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www.marinelink.com 17mature anode depletion was caused by an insufficient anode mass which increased the current demand required per anode. Stork was required to de-sign a new CP system, and given the short operating window, the company took the decision to design a system remotely from Aberdeen, UK that could be assembled and installed lo-cally using only the anodes that were readily available at the yard in West Africa.The vessel?s original CP design con- sisted of a sacrificial anode system in conjunction with a coating system instead of an ICCP system. To design the new system the vessel was ana-lyzed in various categories, including the hull, rudder, nozzle, sea chests and thrusters, as each component pre-sented a different design challenge. The possible electrical discontinuity between components meant that the CP systems on areas that were not electrically connected had to operate independentlyAfter considering a range of op-tions, a conservative current density was used to allow for a more rapid breakdown in coating than normal. This decision was also influenced by the fact the vessel normally operates in warmer tropical water which can result in a higher corrosion rate. The conservative current den-sity employed in the design ensured that the previous high depletion rate would not occur within the system?s intended design life.CP system?s based on a predomi- nantly zinc alloy, which was designed to last for three years, and a predomi-nantly aluminium alloy, designed to last for five years, were developed to allow the subsea services provider to draw a comparison on the most effec- tive design based on what was avail-able on site. The aluminium based system provides a greater current be drawn for a longer period of time per unit mass than zinc. As well selecting the most effective materials for a CP system, the place- ment of the anodes is equally impor- tant to ensure that the entirety of the vessel?s hull and associated compo- nents are sufficiently protected. De- tailed docking plans were also devel-oped to enable the on-site engineers to correctly position the anodes onto the ships hull for optimum efficiency. The vessel has now been re-fitted and has returned to operational capac-ity. Regular dip cell potentials will be taken around the hull to monitor the effectiveness of the system and ensure it is operating as planned.Conclusion With significant global demand for services such as survey, IRM and div- ing, vessels that are dry docked for corrosion related issues can result in significant loss of revenue for sub-sea service providers. CP systems are a cost-effective corrosion mitiga- tion solution, however, they must be designed and installed correctly to operate for the expected design life. Failure to do this can result in prema-ture corrosion and a remedial design being required. Stork developed a CP solution remotely and within a tight operating window for a survey vessel that helped prevent any further unnec-essary downtime due to a corrosion related issue.MR #3 (10-17).indd 17MR #3 (10-17).indd 172/26/2013 3:44:14 PM2/26/2013 3:44:14 PM
